1. The Field of the Invention
The present invention generally relates to x-ray tube devices. More specifically, the present invention relates to an x-ray tube wherein the need for a fluid-filled outer housing is eliminated.
2. The Relevant Technology
X-ray generating devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly employed in areas such as medical diagnostic examination, therapeutic radiology, semiconductor fabrication, and materials analysis.
Regardless of the applications in which they are employed, most x-ray generating devices operate in a similar fashion. X-rays are produced in such devices when electrons are emitted, accelerated, then impinged upon a material of a particular composition. This process typically takes place within an x-ray tube located in the x-ray generating device. The x-ray tube generally comprises a vacuum enclosure, a cathode, and an anode. The cathode, having a filament for emitting electrons, is disposed within the vacuum enclosure, as is the anode that is oriented to receive the electrons emitted by the cathode. The vacuum enclosure may be composed of metal (such as copper), glass, ceramic, or a combination thereof, and is typically disposed within an outer housing. The entire outer housing is typically covered with a shielding layer composed of lead for preventing the escape of x-rays produced within the vacuum enclosure. In addition, a cooling medium, such as a dielectric oil, is typically disposed in the volume existing between the outer housing and the vacuum enclosure in order to dissipate heat from the surface of the vacuum enclosure. The oil may be cooled by circulating it to an external heat exchanger via a pump and fluid conduits disposed in the outer housing.
In operation, an electric current is supplied to the cathode filament, causing it to emit a stream of electrons by thermionic emission. In anode grounded x-ray tubes, a high negative electric potential is placed on the cathode while the anode is electrically grounded. This causes the electron stream to gain kinetic energy and accelerate toward a target surface disposed on the anode. Upon approaching and striking the target surface, many of the electrons convert their kinetic energy and either emit, or cause the target surface material to emit, electromagnetic radiation of very high frequency, i.e., x-rays. The specific frequency of the x-rays produced depends in large part on the type of material used to form the anode target surface. Target surface materials having high atomic numbers (xe2x80x9cZ numbersxe2x80x9d), such as tungsten carbide or TZM (an alloy of titanium, zirconium, and molybdenum) are typically employed. The x-rays are then collimated so that they exit the x-ray device through windows disposed in the vacuum enclosure and outer housing, and enter the x-ray subject, such as a medical patient.
A recurrent problem encountered with the operation of x-ray tubes deals with the removal of heat therefrom. In general, only a small percentage of the electrons that impact the anode target surface during x-ray production do, in fact, produce x-rays. The majority are instead absorbed into the anode target surface and surrounding areas, thereby creating large quantities of heat. This heat must be continuously and reliably removed from the anode and surrounding areas in order to prevent damage to critical tube components. To the extent that the heat is efficiently removed, less thermal and mechanical stress is imposed upon the x-ray tube, and its operation and performance will be enhanced. If the heat is allowed to buildup to detrimental levels, however, it can damage the anode and/or other tube components, and can reduce the operating life of the x-ray tube and/or the performance and operating efficiency of the tube.
Many approaches have been implemented to help alleviate the problems created by heating within the x-ray tube. For instance, in many x-ray tubes the anode, which typically comprises a substrate and a target surface disposed thereon, is formed in the shape of a disk. The rotary anode (also referred to as the rotary target or the anode disk) is then mounted on a supporting shaft and rotor assembly that can then be rotated by some type of motor, such as a stator. During operation of the x-ray tube, the rotary anode is rotated at high speeds, which causes successive portions of the target surface to continuously rotate into and out of the path of the electron beam produced by the cathode filament. In this way, the electron beam is in contact with any given point on the target surface for only short periods of time. This allows the remaining portion of the surface to cool during the time that it takes to rotate back into the path of the electron beam, thereby spreading the heat absorbed by the anode.
While the rotating nature of the anode reduces the amount of heat present at the target surface, a large amount of heat is still absorbed by the anode substrate, the rotor assembly, the cathode, and other components within the vacuum enclosure. This heat must be continuously and reliably removed to prevent damage to the tube (and any other adjacent electrical components) and to increase the x-ray tube""s efficiency and overall service life.
One approach has been to place the vacuum enclosure within an outer housing, as mentioned above. This outer housing must serve several functions. First, it must act as a radiation shield to prevent radiation leakage resulting from the production of x-rays within the vacuum enclosure. To do so, the can must include a radiation shield, which must be constructed from some type of dense, x-ray absorbing metal, such as lead. Second, the outer housing serves as a container for a cooling medium, such as a dielectric oil, which surrounds and envelops the vacuum enclosure, and which may be continuously circulated by a pump about the outer surface thereof As heat is emitted from the x-ray tube components (anode, support shaft, etc.), it is radiated to the outer surface of the vacuum enclosure, and then at least partially absorbed by the dielectric oil. The heated oil is then passed to some form of heat exchange device, such as a radiative surface, and then cooled. The oil is then recirculated by the pump back through the outer housing and the process repeated.
While useful as a heat removal medium and/or as an electrical insulator, the use of oil and similar liquid coolants/dielectrics that surround and envelop the vacuum enclosure can be problematic in several respects. For example, use of large amounts of cooling fluid adds complexity to the construction and operation of the x-ray generating device. Use of fluid to envelop the vacuum enclosure requires that there be an outer housing as outlined above to retain the fluid. This outer housing must be constructed of a material that is capable of blocking x-rays, and it must be large enough to be completely disposed about the inner evacuated housing to retain the cooling fluid. This increases the cost and manufacturing complexity of the device. Also, the outer housing requires a large amount of physical space, resulting in the need for a larger x-ray generating device. Similarly, the space required for the outer housing reduces the amount of space that can be utilized by the inner vacuum enclosure, which in turn limits the amount of space that can be used by other components within the x-ray tube. For example, the size of the rotating anode is limited; a larger diameter anode is desirable because it is better able to dissipate heat as it rotates.
In light of the above discussion, therefore, a need exists for an x-ray tube that eliminates the problems associated with fluid-filled outer housings. Further, a need exists to provide an x-ray tube whereby sufficient cooling of the vacuum enclosure is efficiently attained, thereby improving the performance and longevity of the x-ray tube. Moreover, an x-ray tube having a simple construction and flexible design would be an advancement in the art.
In accordance with the needs outlined above, an improved x-ray tube is provided wherein the housing thereof comprises a unitary vacuum enclosure in which is disposed the cathode, anode, and associated components. The heat created by components of the present x-ray tube is cooperatively dissipated by way of limited fluid and air cooling systems. In this way, problems associated with an outer housing and a cooling fluid disposed therein are avoided.
Generally, the present x-ray tube comprises an adapter plate to which is connected a cathode assembly and an anode housing. These three components are hermetically attached such that they form the unitary vacuum enclosure of the tube. The vacuum enclosure is designed in such a way as to not require a cooling fluid to envelop it; rather a directed fluid cooling system is combined with an air cooling system to efficiently cool the x-ray tube components during operation.
In one presently preferred embodiment, the present x-ray tube comprises an adapter plate composed of stainless steel. A first hole is defined on the adapter plate to receive a portion of a cathode assembly, which comprises a filament and a cathode shield. A cathode housing is preferably disposed about the cathode assembly. The cathode assembly and adapter plate are sealably attached to one another, with the cathode assembly being disposed on a first side of the adapter plate. On a second side of the adapter plate, an anode housing, in which is disposed an anode assembly, is sealably attached. The anode assembly is rotatably supported within the anode housing via a support shaft, which is in turn received by and fixedly attached to a second hole defined in the adapter plate.
A circular cavity is preferably defined on the second side of the adapter plate. The inner surface of the cavity defines a volume that is sized to receive therein a portion of the target surface of the rotary anode. A window is disposed in a hole defined on the edge of the adapter plate such that x-rays produced on the target surface of the rotary anode are emitted through the window.
Specific portions of the x-ray tube of the present invention are cooled by a fluid, such as water, thereby dissipating heat from those areas of the tube where heat buildup is most likely to occur. Unlike the cooling that occurs with fluid-filled outer housings, however, the fluid cooling system of the present invention utilizes fluid passageways defined in portions of the cathode and the anode housing to direct the cooling fluid. During tube operation, heat produced in the anode and cathode portions of the present x-ray tube is conducted to the regions immediately surrounding the fluid passageways. The heat is then absorbed by the circulating cooling fluid and removed from the tube by a pump. The heated fluid then enters a cooling unit, such as a heat exchanger or radiator, where the fluid is cooled and conditioned before being re-circulated into the fluid passageways within the x-ray tube.
In addition to fluid cooling, the present x-ray tube also utilizes air cooling to remove heat from tube components such as the stator. In a preferred embodiment, a fan shroud is disposed about the stator, which in turn is disposed about and affixed to a portion of the anode housing. The shroud has defined in its outer surface air inlet holes and air outlet holes. The holes provide a supply and escape for air that is circulated past the stator by way of a fan disposed near the bottom of the fan shroud. Heat that is produced by the stator during tube operation is transmitted to the air circulating past it, which air then exits the shroud via the outlet holes.
Portions of the anode housing, adapter plate, and cathode housing are preferably shielded to prevent the escape of x-rays from within the vacuum enclosure. Such shielding may be provided by a layer of lead or other suitable material disposed on the exterior of the vacuum enclosure.
Alternative embodiments include modifications to the adapter plate such that the window is disposed in the anode housing instead of the plate, and disposing an extension ring between the anode housing and adapter plate in order to more easily join the two components.
The improved x-ray tube of the present invention is simpler and smaller than previous tubes having a fluid-filled outer housing. The simplicity of the present tube reduces manufacturing costs, while its smaller size provides additional options with respect to the physical placement of the tube in a particular application, or possible enhancements thereto (such as the placement therein of a larger diameter anode) that were impossible before because of space restrictions within the tube caused by the outer housing and the cooling fluid disposed therein. Further, the present x-ray tube is lighter than the prior design, which improves ease of handling and operating. Though features of the present invention are preferably directed to x-ray tubes having electrically grounded anodes, other x-ray tube configurations may also benefit where a fluid-filled outer housing is not desired.
These and other features of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.